Support arrays in fallback synthetic source implementation (#108878)

This PR extends #108222 adding support for arrays.

docs/changelog/108878.yaml

@@ -0,0 +1,5 @@
+pr: 108878
+summary: Support arrays in fallback synthetic source implementation
+area: Mapping
+type: feature
+issues: []

modules/mapper-extras/src/test/java/org/elasticsearch/index/mapper/extras/SearchAsYouTypeFieldMapperTests.java

@@ -827,12 +827,25 @@ protected boolean supportsIgnoreMalformed() {
     @Override
     protected SyntheticSourceSupport syntheticSourceSupport(boolean syntheticSource) {
         return new SyntheticSourceSupport() {
+            @Override
+            public boolean preservesExactSource() {
+                return true;
+            }
+
             public SyntheticSourceExample example(int maxValues) {
-                String value = rarely()
+                if (randomBoolean()) {
+                    var value = generateValue();
+                    return new SyntheticSourceExample(value, value, this::mapping);
+                }
+
+                var array = randomList(1, 5, this::generateValue);
+                return new SyntheticSourceExample(array, array, this::mapping);
+            }
+
+            private Object generateValue() {
+                return rarely()
                     ? null
                     : randomList(0, 10, () -> randomAlphaOfLengthBetween(0, 10)).stream().collect(Collectors.joining(" "));
-
-                return new SyntheticSourceExample(value, value, this::mapping);
             }
 
             private void mapping(XContentBuilder b) throws IOException {

modules/mapper-extras/src/yamlRestTest/resources/rest-api-spec/test/search-as-you-type/30_synthetic_source.yml

@@ -30,6 +30,13 @@ setup:
         body:
           a_field: null
 
+  - do:
+      index:
+        index: test
+        id: "3"
+        body:
+          a_field: ["quick brown", "fox", "jumps"]
+
   - do:
       indices.refresh: {}
 
@@ -46,3 +53,10 @@ setup:
         id: "2"
 
   - match: { _source.a_field: null }
+
+  - do:
+      get:
+        index: test
+        id: "3"
+
+  - match: { _source.a_field: ["quick brown", "fox", "jumps"] }

server/src/main/java/org/elasticsearch/index/mapper/DocumentParser.java

@@ -625,7 +625,10 @@ private static void parseNonDynamicArray(
     ) throws IOException {
         // Check if we need to record the array source. This only applies to synthetic source.
         if (context.mappingLookup().isSourceSynthetic() && context.getClonedSource() == false) {
-            if ((mapper instanceof ObjectMapper objectMapper && objectMapper.storeArraySource()) || mapper instanceof NestedObjectMapper) {
+            boolean storeArraySourceEnabled = mapper instanceof ObjectMapper objectMapper && objectMapper.storeArraySource();
+            boolean fieldWithFallbackSyntheticSource = mapper instanceof FieldMapper fieldMapper
+                && fieldMapper.syntheticSourceMode() == FieldMapper.SyntheticSourceMode.FALLBACK;
+            if (storeArraySourceEnabled || fieldWithFallbackSyntheticSource || mapper instanceof NestedObjectMapper) {
                 // Clone the DocumentParserContext to parse its subtree twice.
                 Tuple<DocumentParserContext, XContentBuilder> tuple = XContentDataHelper.cloneSubContext(context);
                 context.addIgnoredField(

test/framework/src/main/java/org/elasticsearch/index/mapper/MapperTestCase.java

@@ -1092,6 +1092,14 @@ private Object expectedParsedForBlockLoader() throws IOException {
     public record SyntheticSourceInvalidExample(Matcher<String> error, CheckedConsumer<XContentBuilder, IOException> mapping) {}
 
     public interface SyntheticSourceSupport {
+        /**
+         * @return True if synthetic source support is implemented to exactly store the source
+         *         without modifications.
+         */
+        default boolean preservesExactSource() {
+            return false;
+        }
+
         /**
          * Examples that should work when source is generated from doc values.
          */
@@ -1177,7 +1185,7 @@ public final void testSyntheticSourceMany() throws IOException {
             ) {
                 for (int i = 0; i < count; i++) {
                     if (rarely() && supportsEmptyInputArray()) {
-                        expected[i] = "{}";
+                        expected[i] = support.preservesExactSource() ? "{\"field\":[]}" : "{}";
                         iw.addDocument(mapper.parse(source(b -> b.startArray("field").endArray())).rootDoc());
                         continue;
                     }
@@ -1236,13 +1244,16 @@ public final void testSyntheticSourceInObject() throws IOException {
     public final void testSyntheticEmptyList() throws IOException {
         assumeTrue("Field does not support [] as input", supportsEmptyInputArray());
         boolean ignoreMalformed = supportsIgnoreMalformed() ? rarely() : false;
-        SyntheticSourceExample syntheticSourceExample = syntheticSourceSupport(ignoreMalformed).example(5);
+        SyntheticSourceSupport support = syntheticSourceSupport(ignoreMalformed);
+        SyntheticSourceExample syntheticSourceExample = support.example(5);
         DocumentMapper mapper = createDocumentMapper(syntheticSourceMapping(b -> {
             b.startObject("field");
             syntheticSourceExample.mapping().accept(b);
             b.endObject();
         }));
-        assertThat(syntheticSource(mapper, b -> b.startArray("field").endArray()), equalTo("{}"));
+
+        var expected = support.preservesExactSource() ? "{\"field\":[]}" : "{}";
+        assertThat(syntheticSource(mapper, b -> b.startArray("field").endArray()), equalTo(expected));
     }
 
     public final void testSyntheticEmptyListNoDocValuesLoader() throws IOException {